Molecular genetics of C4B deficiency in IgA nephropathy

The fourth component of complement (C4) occurs in two functionally distinct isotypes, C4A and C4B. The two closely linked genes are located on chromosome 6p, between HLA-B and -DR. Several reports have established complete C4B deficiency as the major genetic risk factor for IgA nephropathy (RR = 6.5; p = 0.0004). It is not clear whether this association derives from immune dysfunction related to the absent isotype or from another disease susceptibility gene closely linked to C4B. To help distinguish between these mechanisms, we examined the molecular basis of complete C4B deficiency in five patients with IgA nephropathy and eight healthy individuals. C4 and Bf protein typing were performed by immunofixation electrophoresis of plasma. Genomic DNA was digested with several restriction enzymes, chosen to produce informative restriction fragment length polymorphisms (RFLPs). After electrophoresis and Southern blotting, digests were hybridized to a series of cDNA probes specific to the 5' and 3' ends of the C4 genes, the C4d region, and the adjacent 21-hydroxylase genes. Availability of DNA from family members allowed assignment of RFLPs to specific haplotypes. The 10 C4B-deficient IgA nephropathy-associated haplotypes displayed seven different protein phenotype/RFLP patterns. Three haplotypes consisted of the common C4B/21-hydroxylase deletion on the Bf*S, C4A*3, C4B*Q0 complotype. Two haplotypes were characterized by the C4A*3,2 duplication, with two C4 genes present but a C4A protein being produced by the gene at the usual C4B locus. All of the remaining haplotypes had unique Bf, C4, and 21-hydroxylase patterns. C4B-deficient IgA nephropathy patients display a variety of molecular genetic bases for their protein deficiency. This observation speaks against linkage of C4B deficiency with a locus encoding disease susceptibility and supports a primary role for the complement abnormality in this disease.